59. A new mutant called early ripening (EL) is identified in a plant. The wild type phenotype is late ripening (LR). Further DNA marker(s) is/are observed to be polymorphic between EL and LR plants. A cross was made between pure lines of LR and ER. The F1 progeny was test crossed and its progeny was analyzed. The parental, F1 and progeny of test cross were also analyzed for the DNA marker. The table below summarizes thephenotype of the progeny and the pattern of DNA marker observed in each case:
Based on the above information, the following statements were made:
A. LR is dominant to ER
B. The DNA marker used is a dominant marker
C. The DNA marker is linked to the phenotype
Which of the above statements are correct?
(1) A only (2) A and B only
(3) A and C only (4) A, B and C only
Reconstructing the question (ER vs LR)
From the image, the experiment can be summarized as:
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A new mutant early ripening (ER) is found; wild type is late ripening (LR).
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Pure lines of ER and LR are crossed → F₁ is LR only (all plants LR).
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F₁ is test‑crossed and among the test‑cross progeny:
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Phenotypes: 52 LR and 48 ER (≈1:1 ratio).
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A DNA marker is polymorphic between ER and LR and its band pattern is followed in parents, F₁ and test‑cross progeny.
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The final row shows that test‑cross progeny with LR and ER phenotypes each receive about half of their marker alleles from each parent (counts ~26, 27, 23, 25), consistent with 1:1 segregation and linkage to the trait.
Given this, evaluate:
A. LR is dominant to ER
B. The DNA marker used is a dominant marker
C. The DNA marker is linked to the phenotype
Options:
(1) A only (2) A and B only (3) A and C only (4) A, B and C only
Step‑wise reasoning
1. Dominance of LR vs ER (Statement A)
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Parents: one pure LR, one pure ER.
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F₁ phenotype: LR only.
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When a cross between pure lines yields F₁ all showing one phenotype, that phenotype is dominant.
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Therefore LR is dominant over ER.
✅ Statement A is TRUE.
2. Nature of the DNA marker (Statement B)
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The marker is described as “DNA marker(s) polymorphic between ER and LR plants”.
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On the gel pattern (from the slide), presence of a particular band is scored; individuals either show that band or they do not.
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Such scoring (band present vs absent) with segregation in the cross is characteristic of a dominant marker system (e.g., RAPD, AFLP, dominant SSR scoring pattern).
Thus the marker behaves as a dominant marker.
✅ Statement B is TRUE.
3. Linkage of marker to the phenotype (Statement C)
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In the test‑cross progeny, there are roughly equal numbers of LR and ER plants (52:48 ≈ 1:1), indicating the F₁ was heterozygous for the ripening locus.
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Marker band pattern in the progeny co‑segregates with the phenotype in an approximately 1:1:1:1 distribution split by parental contributions, which is what is expected when a marker is linked (but not necessarily perfectly) to the trait locus rather than assorting completely independently.
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The counts contributed from each parent (26, 27, 23, 25) show that particular marker bands tend to travel with specific phenotypes, evidencing linkage between the marker and the ER/LR locus (not random assortment).
✅ Statement C is TRUE.
Since A, B and C are all correct, the correct option is (4) A, B and C only.
Quick evaluation of options
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Option (1) A only – Incorrect; B and C are also correct.
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Option (2) A and B only – Incorrect; it omits true linkage statement C.
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Option (3) A and C only – Incorrect; it omits true marker‑type statement B.
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Option (4) A, B and C only – Correct; all three statements match the genetic evidence.
Introduction
Genetic analysis of new mutants such as early ripening (ER) versus late ripening (LR) often combines classical crosses with polymorphic DNA markers to determine dominance, marker type and linkage. Understanding F₁ phenotypes, test‑cross segregation and marker band patterns allows precise inference about how a molecular marker relates to a visible trait. This solved MCQ on early and late ripening illustrates how to deduce dominance, identify a dominant DNA marker and recognize that the marker is linked to the ripening locus.
Explanation of each statement
Statement A: LR is dominant to ER
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Crossing pure ER with pure LR yields F₁ plants that are all LR, meaning the LR allele masks the ER allele in heterozygotes; by definition, LR is dominant and ER is recessive.
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The 1:1 LR:ER ratio in the test‑cross progeny further confirms the F₁ was heterozygous for a single major locus, consistent with simple dominance of LR.
Statement B: DNA marker is dominant
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The DNA marker is described as polymorphic between ER and LR, and the electrophoresis pattern records presence or absence of a specific band, a classical dominant‑marker scoring (band vs no band).
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Under this model, a single copy of the marker allele produces the band (marker “present” phenotype), so the marker behaves as a dominant marker.
Statement C: DNA marker is linked to the phenotype
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If the marker and phenotype loci assorted independently, marker band presence would not correlate with ER or LR status in the test‑cross progeny and would show a purely random association.
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Instead, the observed progeny numbers and the way marker bands track with ER or LR indicate that the marker allele tends to co‑inherit with the ripening allele, signifying that the marker locus is linked to the ER/LR locus on the same chromosome.
Final answer
Because all three statements A, B and C correctly describe the genetics of the cross and the marker behaviour, the correct choice is (4) A, B and C only.
